Direct coupling two-stage transistor amplifier



April s, 195s JEAN-MARIE MOULQN DIRECT COUPLING TWO-STAGE TRANSISTOR AMPLIFIER Filed Feb. 26., 1954 5 Sheets-Sheet 1 April 8, 1958 'JEAN-MARIE MouLoN 2,830,134

DIRECT COUPLING TWO4STAGE TRANSISTOR AMPLIFIER Filed Feb. 26, 1954 v 5 Sheets-Sheet 2 April 8, 1958 JEAN-MARIE MouLoN l 2,830,134

DIRECT COUPLING Two-STAGE TRANSISTOR AMPLIFIER Filed Feb. 26, 1954 5 Sheets-Sheet 3 4:21 Fig. 5

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DIRECT COUPLING Iwo-STAGE' TRANSISTOR AMPLIFIER Filed Feb. 2s, 1954 5 sheets-sheet 4 April 8, 1958 JEAN-MARIE MoULoN '2,830,134

DIRECT COUPLING TWO-STAGE TRANSISTOR AMPLIFIER Filed Feb. 25, 1954 5 sheets-sheet 5 Fig. 8

0 1 2 .z 4 mA United States Patent O DIRECT COUPLING TWO-STAGE TRANSISTOR AMPLIFIER Jean-Marie Moulon, Paris, France Application February 26,1954, Serial No. 412,674 Claims priority, application France May 9, 1953 Z Claims. v(Cl. 179-171) The present invention relates to a direct coupled twotransistor amplifier` assembled in such avmanner thatthe collector electrode of the rst transistor is connected directly with the emitter electrode ofthe second transistor, the base electrodes ofV both transistorssbeing connected at a constant potential pointof the mounting by means of suitably chosen impedanc'es, Such anassembly constitutes a cascade type two-stage amplifier. l

An object of the present invention is to obtain a twotransistor amplifier having a substantial current and power gain, and which can be power supplied from a single current source.

Another object of the present invention is to obtain a direct coupled two-transistor amplifier capable of operating as a D. C. amplier. InV particular, the amplifier comprises no interstage transformer and uses a single source of power supply instead of adistinct source for each stage.

It is known that an -assembly of two 'transistors conn ected in cascade, and 'connecting on the one hand the collector electrode of the r'st one with the emitter electrode of the second one, and, on the other hand the base` electrodes with eachother, and with a constant potential point biased by a main current source connected between this point and the collector' electrode of the second transistor, has no amplifying properties. If an electric signal voltage is applied between thebase electrode and the emitter electrode of the rst transistor, and if a load impedance is connectedrbetween the base electrode and the collectorelectrode of thesecond transistor, it is necessary inorder to haveu an. amplification, that the emitter-electrode of the iirst transistor be positivelyv biased with respect to its collector electrode, byl means of an auxiliary current source 'and' 'thata similar bias be applied to the emitter electrodeY of the second transistor with respect to its collector electrode by means of the mainV current source connected as explained above. This leads again tothe drawbacks of D; C. tube amplifiers which require one current source for each stage.

According to the. invention, 'the' device whichjhas just been described is improved by the insertion, in the circuit of the base electrode of the second transistor, ofva socalled feedback resistance which lis chosen larger than the transfer return resistance representing the action yof the currenttin the'circuit of the collector electrode Yon the emitter-'electrode voltage as this action would existin the absence of said feedback resistance. The elfect of the insertion of this resistance is to space more widely from-one another the characteristic curves representing, for various values of the` output current ofthe second transistor, the input voltage plotted against the input current supplied to said second transistor.

The result, asrwill be shown hereinafter, is -that there corresponds, to a. given variation of the input current supplied to the assemblyofthel two transis-tors, a larger variation of the output current ofthe assembly than in case no resistance-isv insertedin the base electrode circuit of the second transistor. yIn addition, the presence of ance in its base electrode circuit;

2,830,134 Patented Apr. 8, 1958 ICC the resistance has the effect of causing, through the assembly of the amplifier, a distribution of direct potentials which obviates the need foran auxiliary power supply source. Y

The invention having been briefly delined will now be described in greater detail with reference to the appended drawings, wherein: 1 l

Figure l shows a direct coupled two-transistor amplifier according to the invention; l

Figure 2 shows the characteristic curves giving, for various values ofthe output current from'thesecond transistor, the input voltage as a function of thecinput` current to said second transistorin case there is no resist- Figure 3` represents the characteristicl curves giving, for various values of the input current to the first transistor, the output voltage as a function of the output current from this rst transistor;

Figure 4 is a superposition, made ,inV conditions which will be explained hereinafter, of the characteristic curves of'Figures 2 and 3. v f Figure 5 .shows theV characteristic curves givingfor variousV values of the output current from the second transistor having ar resistance in its base electrode circuit, the input voltage asva function of the input current tof said ASecond transistor; y `Figure 6v isa superposition made in conditions which will be set forth hereinafter, of the characteristic curves of Figures l3 and 5; v

Figures 7 i and 8 represent, respectively the input and output voltage-current characteristics of the two-transistor amplier of Figure l; and y Figure 9 represents an assembly of two transistors according tothe invention,.used for controlling a telephone relay or repeater.`A y. l As'shown in Figure'l; the` two-stage transistor ampliiier according to the invention, represented .inv this figure-,comprises essentially twoV transistors 1 and 2,- means comprisingvinput terminals?, and 4 for applying a signal voltage to be amplified between 'the ernit'te'r electrode of transistor i' and the base electrode ofl this transistor considered as av constant potential point ofthe assembly, a direct connection 8 between the collector electrode of transistor llv and the emitter electrode :of transistor 2, a resistance 10 connecting thebas'e electrode of transistor 2 with the base electrode of transistor l, a

source of direct bias voltage 7 connected by one of its terminals to said constant potential point and means comprising output terminals 5 and 6 Vfor connecting an outer load impedance, on'the one hand with the collector' electrode of transistor 2 and on the other fhand with the other terminal of source '7. s f

Theoperation of the amplifier of Figure l will first be explained as regards its D. C. characteristics, after which itsoperaticn will be considered for small alternating voltagelvariations and'currentvariau'ons occurring' in the vicinity of theA D. C. voltage and current conditions corresponding to predetermined power supply' conditions. i

All voltages, hereinafter, will be referred to terminal 4 and to the base electrode in -transistor 1, considered as a zero potential point.

' Designating by Ve1 and lel the voltage and current in the circuit of the emitter electrode of the first transistor, by V01, and Ic1 the voltage Vand current in the circuit ofthe collector electrode Vof the same transistor,

Figure 2 shows the group of Ve2-Ie2 characteristicsY have been superposed by bringing the axes Ic1 and 12 to coincidence, since the currents are the samein the coll lector electrode ofthe-transistor 1 and in the emitter electrode of the transistor 2, and by opposing the axes Vc1 and V1,2 in order to take into account the irst relationship 1).

If a certain load impedance is connected to the output Vterminals -`6,` there corresponds thereto, in the V,2 and 1,2 curve group of Figures 2 and 4, a certain load curve` 12 which depends on the value of the impedance of the connectedload and on the electromotive force of the source 7. Whichever this curve may be, it is obvious from Figure 4, there corresponds that to any variation of Ie1 on this curve only a very low variation of Ic2 as the Ve2-I2 characteristics intersect the Vel-41.1 characteristics in the area close to the origin, where the latterV are not yet distant from one another. This is due to the fact that the transistor 1 is not biased normally. Y

In order that a variation of Ie1 may cause a substantial variation of 162, it is necessary to increase the spacing between the characeristic V22-Ie2 curves corresponding to different values of L12. This result is` obtained, according to the invention, by inserting a feedback resistance of the order of 5,000 ohms in the circuit of the base electrode of transistor 2 (Fig. l). Such a resistance increases the action of the current in the collector electrode circuit on the voltage of the emitter electrode, and consequently has the etectof increasing the spacings between the characteristics of the Ve2-Ie2 network. This new network is shown in Figure 5, and it is also shown inFigure 6, in superposition on the network of characteristics Vel-Ici. By comparing Figures 4 and 6, it is found that the two networks I/l--Ic1 and K12-[e2 have a wide area in common in the rst quadrant, numbered` (I) in the case of Figure 6. The loady curve represented at 12 oiers-a negative slope due to the positive feedback introduced by the resistance inserted in the base circuit of the transistor 2.

A variation in L,1 is then reflected on the current 11,2. For example (Fig. 6), for Ie1=0, [c2-:1.5 ma. and for 151:1 ma., 122:3.5 ma. The current gain may be increased, generally, by modifying the slope of the curve 12', i. e. by matching as well as possible the value of the load impedance connected toV terminals 5 and 6.

The advantage concerning the current gain obtained by the set .up of Figure 1 as compared to the setup including no `resistance 10, for the case of small voltage and current variations about a working point in the system will be made clear hereinafter. .t

It will be assumed that the transistors 1 and 2 have average characteristics, for` instance an input resistance R11=3O0 ohms, an output resistance R22=l0,000 ohms and transfer resistances R12 and R21 translating respectively Vthe action of the circuit of the collector electrode on the voltage of the emitter electrode and the action of theremitter electrode circuit on the voltage of the collector electrode, respectively equal to 100 and 20,000 ohms. As a result, the current gain of each transistor a=R21/R11 is equal to 2. It will be assumed further that the resistance 10 inserted in the base electrode circuit of the second transistor has, for instance, a valueV R equal to 5,000 ohms. Under these conditions, the impedance matrix of the rst transistor may be written:

R11 R12 300 100 l (2) Rn t?22 k 20,000 10,000

and that of the second transistor is obtained by adding the quantity R, i. e. 5,000'ohms toA all terms ofthe matrix (2) which'gives neglecting 300 ohms and 100 ohms as compared to 5,000 ohms:

R11-FR R12-PRH Rn R12 5,000 1 5,000I

R21+R Riza-R R'21 411e/22 25,000 15,000

Taking'into account the fact that the characteristic matrix of two four-terminal networks connected in chain is equal to the product of the characteristic matrices of the two four-terminal networks of the chain and the relationship existing between a characteristic matrix and an impedance matrix for a given network, and if:

S21 vS22 is the impedance matrix for the chain, we have:

S R Ruft.. 20,000

1" 1 Rn-i-Ru 10,000+5,000

q RmRiz 100X5,000 du RarlR'il 10,00l5,000 122.1231 20,000 25,000 "Ra-i-Rn* 10,000+5,000 33000 Ohms R'121t' y 5,000 25,000 R+R'n 15000 10,000+5,000

=300 Ohms 33 ohms =6,700 ohms whence:` Y

(3) 'S11` Si: 160 33 s sfaaooo 6,700 (s) Comparing the impedance matrix (3) withV the impedance matrix (2)V of the transistors 1 and Z, we find that: "-the current gain passed from a=R21/R22=2 to a'=S21/S22=5 -the short circuit stability coefcient 6 is lowerfor the assemblylof the two transistors `than for a single transistor since R12R21 l00 20,000

y for a single transistor, While:

There is plotted, for the above numerical values, in Figure 8, the network of characteristics giving V (output voltage) as a function of I (output current) of the twotransistor assembly for various values of the input current i. On Figure 7 is plotted the network of characteristics giving v (input voltage) as a function of (input current) of this same assembly `for various values of the output current I. On these networks of characteristics, which are similar to those for a single transistor, may be found the above calculated performances. The output impedance given by the slope of the characteristics of Figure 8 is `of the order of 7,000 ohms. The current gain is of the order of 5; the characteristics corresponding to Y=0 .and =0.25'ma. for instance, are spaced by 1.25 ma. -parallel to the I axis. The power gain G is important; it is given,` assuming for instance the twotransistor assembly connected iwith 'a sourcehaving an 5 internal resistance r=160 ohms, and a load resistance r=6,700 ohms, by

G: (T+ T')S21 ]Z= (S11-i' 7') (S22-i- T') 512521 32OX1,40033X33,000 G=37 decibels It appears from the above that the direct coupled twotransistor assembly according to the invention behaves like an improved transistor with important current and power gains. It may be used, therefore, in all set-ups using transistors by a simple substitution for a single transistor, and it may be power supplied from a single current source like the single transistor for which it is substituted.

The two-transistor assembly may be used for subjecting the D. C. in the collector electrode circuit of the second transistor to important power variations derived from small power variations in the input circuit. In Figure 9, 11 represents a relay winding having a 4,000 ohm resistance inserted in the collector electrode circuit of transistor 2, the other reference numerals being the same as in Fig. l. For a power variation at the input of the order of l0 milliwatts, power variations are obtained in the relay circuit of the order of 150 milliwatts, suicient for controlling the relay.

What is claimed is:

1. A two stage transistor amplifier comprising rst and second transistors; each of said transistors having base, emitter and collector electrodes; input terminals for applying a signal voltage to be ampliiied between the base and emitter electrodes of said first transistor; a biasing direct current source for supplying biasing current to said transistors; a utilization circuit; connections for connecting said utilization circuit in series with said source between the collector electrode of said second transistor and the base electrode of said rst transistor, a direct connection between said collector electrode of said first transistor and said emitter electrode of said second transistor; a resistor connected between the respective base electrodes of said rst and second transistors.

2. An amplifier as claimed in claim 1, wherein the resistance value of said resistor is of the same order of magnitude as the internal dynamic collector-base electrode resistance of said second transistor in its normal amplifying condition.

References Cited in the le of this patent UNITED STATES PATENTS 2,585,078 Barney Feb. 12, 1952 2,622,213 Harris Dec. 16, 1952 2,666,817 Raisbeck et al. Jan. 19, 1954 

